As small and light digital still cameras and cellular phones with cameras become used widely, miniaturization of pixels in a solid-state imaging device such as Charge Coupled Device (CCD) or Metal Oxide Semiconductor (MOS) type device has been promoted. With the miniaturization promoted, a light-receiving area above a light-receiving sensor portion (photoelectric conversion portion) becomes small, deteriorating the sensitivity of pixels.
To deal with such deterioration of the sensitivity, a measure in which a microlens is formed above the light-receiving sensor portion has been proposed (JP 4 (1992)-37165A). And a measure in which an optical waveguide is formed above the light-receiving sensor portion also has been proposed (JP 10 (1998)-326885A, JP 2003-46074A, JP 2004-221532A). In this measure, openings in the interlayer insulating film are formed above the light-receiving sensor portion, and optical waveguides made of a material having a higher refractive index than the interlayer insulating film are formed in the openings. The optical waveguides improve the efficiency of condensing light into the light receiving sensor portion.
As a material for making the optical waveguides, silicon nitride having a refractive index of 2.0 has been disclosed (JP 10 (1998)-326885 A). Also, diamond-like carbon (DLC) having a refractive index of about 3.0 and polyimide resin having a refractive index of about 1.7 have been disclosed (JP2003-46074 A, page 4). Also, SiO2 has been disclosed (JP2004-221532 A).
However, the miniaturization of the solid-state imaging device is progressing further recently, and the area of the light receiving sensor portion is becoming smaller. Accordingly, the aspect ratio of the opening formed above the light receiving sensor portion becomes large in the solid-state imaging device equipped with optical waveguides. As a result, coverage of a high refractive index material (material for the optical waveguide) embedded in the openings deteriorates. The deterioration of the coverage may lead to air gaps such as voids in the optical waveguide being generated. When voids are formed in the optical waveguide, it causes deterioration such as reduction of sensitivity and variation of sensitivity.
Such deterioration becomes significant in the case of an optical waveguide formed by a vacuum deposition such as a silicon nitride film and a DLC film. On the other hand, in the case of an optical waveguide formed by application method, that is a polyimide resin for example, the embedding can be conducted better than that in the optical wave guide formed by a vacuum deposition. However, as the aspect ratio is getting larger, the embedding becomes more difficult.
In the meanwhile, the larger the difference between the refractive index of the optical waveguide and that of the interlayer insulating film is, the higher the light-condensing efficiency by the optical waveguide becomes. Therefore, it is preferable that the refractive index of material for the optical wave guide is as high as possible. However, the refractive index of resins is about 1.6 to 1.7 at the highest. Accordingly, a method for making the optical waveguide with a material having a higher refractive index and a better film-forming property is desired.